Much research on advanced modes of combustion such as Homogeneous Charge Compression Ignition (HCCI) has indicated that it is very difficult to find a fuel that is capable of using such modes over the full load and speed range of an engine. At low loads and speeds, diesel fuel is suitable because of its low auto ignition temperature. However, at high loads and speeds where the cylinder temperature will be higher, diesel fuel can ignite too far before top dead centre and burn too quickly. This results in low efficiency, excessive cylinder pressures and high engine noise. Petrol or ethanol is a more suitable fuel for such conditions because of its higher auto ignition temperature. However, a higher auto ignition temperature means that it is difficult to obtain compression ignition with these fuels at low engine speeds and loads.
A solution to this problem would be to have a fuel injection system able to change between different fuels for different operating conditions. With the currently known technology this would entail having two separate injection systems with dedicated injector sets, inlet metering valves, pressure regulators and fuel pumps. This would be very expensive for automotive use and may give rise to space and/or weight problems.